Bleaching



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m G 55 grinders usually runs between 100 and 212 F. U

i 20 UNITED STATES PATENT OFFICE x w nmacnmo. 6 9 Wilhelm Hirschkind, Berkeley, Calif., alsignor to Great Western Electra-Chemical Company, a corporation of California I No Drawing. Application August 11, 1938, Serial No. 95,430

5 Claims. (01. 8-2) This invention relates to the bleaching of usually about 140 F., and between 140 and 180 groundwood pulp, particularly of groundwood F. Instead of utilizing the heat at the grinders, pulp suspensions. artificial heat can be resorted to, but this is of In my Reissue Patent No. 19,470, of February course more expensive.

19, 1935, I have disclosed the application of The material utilized to maintain the pH be- 5 hydrosulphite to the bleaching \of groundwood tween 4.5 and 6, and preferably between 5 and 6, pulp. In my application Serial No. 755,929, filed must be chosen with care-a material so strongly December 4, 1934, I disclosed that it was advanacid or alkaline that the pH at the end is carried tageous to bleach groundwood pulp with a hydrobeyond the range specified is not useful. I have 10 sulphite at a pH maintained substantially besuccessfully used as materials for maintaining 10 tween 5 and 6. I have now determined that it the desired pH values diammonium phosphate, is advantageous to bleach groundwood pulp at a sodium bicarbonate, s um borate. The high temperature (of the order of 140 F'.) while borates, bicarbonates phosph es, sulphites, carmaintaining the pH of the groundwood pulp be bonates, acetates and fer'niates of the alkali tween 4.5 and 6.5 and preferably between 5 and 6. metals, including the ammonium radical, are usea I have found that as the pH approaches 4 pink ful, as are calcium sulphite, phosphate and shades became prevalent, while the blues enter as borate. '7 is approached. The amount of the alkaline material used is The advantage of operation at high temperaquite small, being approximately one-fifth the Q tures will be apparent upon considering prior quantity of the hydrosulphite in the case of operations under the controlled pH condition. diammonium phosphate, and about one-fourth While high temperature treatment with a hydrothe quantity in the case of sodium bicarbonate. sulphite gave an effective bleach, the product Since the amount of hydrosulphite employed is was quite yellow. Low temperature bleaching relatively small, from 0.25% to 10% the amount r with a hydrosulphite, while maintaining the pH of the salt utilizedis also small, particularly when between the preferred values of 5 and 6, also gave it is pointed out that 1% of the hydrosulphite a product which was commercially acceptable but usually sumces. which nevertheless included to an appreciable After the bleaching operation, the pulp is extent objectionable yellow shades. In carrying treated in whatever manner desired to finally out the high temperature bleach operation it was convert it into paper. 30

also necessary to wash immediately following the I claim: bleaching operation. 1. Bleaching ground wood at a temperature be- It is my discovery that, if the high temperature tween 100 and 212 F. with a hydrosulphite at a operation be combined with the pH control, the pH substantially between 4.5 and 6.5 while mainproduct secured is more purely white, the obtaining said pH during said bleaching by addition jectionable yellow shades of the cold treatment of an inorganic buffering salt effective to mainor treatment without pH control'being substantain said pH substanti ll constanttially entirely eliminated. 2. A process for treatment of a mechanical In addition, I have found that materials need wood pulp suspension comprising subjecting pulp 40 not be washed immediately following the bleach in said suspension at a temperature of 100 to 40 operation, the objectionable compounds formed 212 F. to reduction by a salt of hydrosulphurous in the high temperature bleach operation, in the acid while maintaining said suspension at a pH absence of pH control, being eliminated when pH substantially between 4.5 and 6.5 by addition of control is exercised. Obviating the necessity of an inorganic salt effective to maintain said pulp the wash eliminates the high cost of equipment substantially at said pH. 5

for this operation. 3. Bleaching ground wood at a temperature be- In carrying out this invention advantage can tween 100 and 212 F. with a hydrosulphite at a be taken of the high temperature produced at the pH substantially between 5 and 6 while mainpulp grinders as the hydrosulphite is added to the taining said pH during said bleaching by addition grinder showers or to the grinder pits. Any salt of an inorganic buffering salt eflective to main- 50 of hydrosulphurous acid, as sodium, potassium tain said pH substantially constant. or zinc hydrosulphite, can be utilized, and the 4. A process for treatment of a mechanical hydrosulphite can be formed in any manner well wood pulp suspension comprising subjecting pulp known in the art. The temperature at the in said suspension at a temperature of 100 to 212 F. to reduction by a salt of hydrosulphurous acid while maintaining said suspension at a pH substantially between 5 and 6 by addition 01' an inorganic salt eflective to maintain said pulp substantially at said pH.

5. A process for bleaching a ground wood pulp comprising subjecting said pulp at a temperature between 100 and 212 F. to the action of a hydrosulphite present in an amount 01' about 1% of the weight of the pulp and while maintaining the pH of said pulp between 4.5 and 6.5 by addition of an inorganic salt present in amount of about 0.25% of the weight of the pulp.

.WILHELM HIRSCHKIND. 

